Integrated circuit random access memory capable of automatic internal refresh of memory array

ABSTRACT

A dynamic random access memory integrated circuit and method includes internal refresh control and an array configured to receive read and write access requests having priority over pending refresh requests, wherein refresh requests are queueable and retired on clock cycles not requiring an access of the array and complete in one clock cycle. No onboard cache memory is required. A method includes: determining within the circuit when one of the banks of the array requires a refresh, prioritizing read and write access requests over pending refresh requests, read access requests initiating an access to the array without determining whether data is available from outside the array, and retiring within a clock cycle one pending refresh request to a bank when that bank has pending refresh requests and does not also require an access of the array on that clock cycle.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of pending U.S. patentapplication “MERGING WRITE CYCLES BY COMPARING AT LEAST A PORTION OF THERESPECTIVE WRITE CYCLE ADDRESSES,” U.S. Ser. No. 09/503,048 filed Feb.12, 2000 now U.S. Pat. No. 6,356,485, having Robert J. Proebsting listedas inventor, which application is a nonprovisional application of U.S.provisional patent application “HIGH PERFORMANCE DYNAMIC RANDOM-ACCESSMEMORY ARRAY ARCHITECTURE AND ASSOCIATED SUPPORTING CIRCUITRY,” U.S.Ser. No. 60/120,032 filed Feb. 13, 1999, having Robert J. Proebstinglisted as inventor. The Ser. No. 60/120,032 application and the Ser. No.09/503,048 application are hereby incorporated by reference in theirentireties for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor memories, andparticularly to architecture and organization of a dynamic random-accessmemory array having automatic internal refreshing of the memory array.

2. Description of Related Art

Semiconductor random-access memory devices or sub-systems using arraysof dynamic memory cells (e.g., 1-transistor/1-capacitor (1T/1C) cells)have consistently provided greater density and lower cost per bit thanthose using static memory cells (e.g., 6-transistor (6T) cells, or4-transistor/2-resistor (4T/2R) cells). However, such dynamicrandom-access memory arrays have historically had lower operationalfrequencies and longer read access times when compared to staticrandom-access memory arrays. Consequently, system designers havetypically chosen dynamic memory arrays (e.g., commercially availabledynamic random access memories, or DRAMs) when high density and low costare required, such as for CPU main memory applications. Conversely,designers have typically chosen static memory arrays when the highestpossible performance is required, such as for cache memory and highspeed buffer applications. Examples of static memory array devices orsub-systems include commercially available static random access memories(SRAMs) and CPU-resident on-board cache memory sub-systems.

The reasons often cited for the lower performance of dynamic memoryarrays include the destructive sensing of all memory cells common to theaddressed word line (encountered in virtually all dynamic memory arrays)and the consequential need to restore data back into each sensed memorycell during the active cycle, the need to equilibrate bit lines andvarious other differential nodes and to precharge various circuit nodesbetween active cycles, and the requirement for periodic refreshing ofall dynamic memory cells.

The need for ever-increasing memory performance at reasonable costcontinues unabated. It is being driven by numerous advances towardhigher frequency and higher speed applications. For example, theincreasing bandwidth demands of computer networking and the internet,the increasing speeds of commercially available processors, and theproliferation of high-frequency wireless communication systems are alldriving the need for higher performance memory subsystems.

In addition to the increased speed requirements of memory systems, thereis also an increasing need for low cost memory made possible by smallmemory cells. The smallest known read/write memory cell is the onetransistor/one capacitor dynamic memory cell. Therefore, there is anunfilled need for memory devices that combine the cost advantages of thedynamic memory cell with the performance normally found in memoriesincorporating larger more expensive SRAM cells. In particular, arefreshing technique and method is needed that meets the goals formemory devices.

SUMMARY OF THE INVENTION

In accordance with the present invention, a random access memoryintegrated circuit automatically refreshes banks in a dynamic memoryarray without requiring an on-board cache memory, thereby using lessspace on the integrated circuit as compared to the space that would berequired if a cache memory were to be included. The disclosed randomaccess memory integrated circuit is responsive to an externally suppliedclock input and includes a dynamic memory array configured in one ormore banks, wherein the array requires periodic refreshing to maintaindata. The random access memory integrated circuit further includes oneor more refresh control circuits generating refresh requests inside therandom access memory integrated circuit. The dynamic memory array isconfigured to receive read and write access requests that have priorityover pending refresh requests. Any pending refresh requests are retiredon a clock cycle not requiring an access of the dynamic memory array.Further, the refresh completes in one clock cycle. The read accessrequests initiate an access to the dynamic memory array without firstdetermining whether data is available from outside the dynamic memoryarray, (i.e., from an internal cache memory) thereby avoiding a delayassociated with such determination.

In one embodiment, each of the one or more banks has a one to onecorrespondence with one of the one or more refresh control circuits,each refresh control circuit controlling refresh of the bank coupledthereto. In embodiments containing two or more banks, during a clockcycle in which one bank of the dynamic memory array executes a read or awrite operation, at least another bank may simultaneously execute arefresh operation. Also, according to an embodiment, during a clockcycle in which one bank executes a read or a write operation, two otherbanks may simultaneously execute refresh operations.

According to a further embodiment containing two or more banks, aseparate address bus is coupled to each bank, each of the banksreceiving an independent address, wherein at least one bit of the reador write access address is used to select the bank for read or writeaccess. The bit is selected from among low order address bits on therandom access memory integrated circuit. In one embodiment, the bit isselected such that the bit changes state frequently. Further, the choiceof bits of the address allows the banks to share read and writeaccessing such that no one bank should require excessive accessingthereby denying that bank the opportunity to meet refresh requirements.Excessive accessing includes accessing the same bank more than 255 reador write cycles of each 256 clock cycles, averaged over many thousandsof clock cycles.

According to one embodiment, the refresh control circuit includes atleast one counter, one refresh request being generated each time thecounter counts a predetermined number of external clock cycles, therefresh requests being generated independent of an elapsed time counter.

According to another embodiment, the random access memory integratedcircuit outputs a refresh flag to indicate that internal refreshrequirements are not being met. The flag is output after a predeterminednumber of refresh requests to any bank is queued.

Referring to each refresh control circuit, in one embodiment, eachrefresh control circuit includes a wrap around refresh request counterthat increments once for each refresh request and a wrap around refreshaddress counter. Each wrap around refresh address counter provides therow address to be refreshed and increments in response to a refresh ofthe bank associated thereto. An inequality between the count in the wraparound refresh request counter and the count in the wrap around refreshaddress counter indicates a pending refresh requirement for that bank.

Still referring to the refresh control circuit, in another embodiment,each wrap around refresh request counter input within its respectiverefresh control circuit is coupled to receive a unique output from adecoder; the decoder input is coupled to receive an output from a wraparound clock cycle counter. Every predetermined number of clock cycles,the wrap around clock cycle counter in combination with the decodercause the decoder to sequentially toggle one of a plurality of outputs.Each decoder output is configured to toggle once at a particular countof the wrap around clock cycle counter, each output to be received by arespective one of the refresh control circuits. The particular countsare the predetermined number of clock cycles apart. The predeterminednumber of clock cycles is more than a maximum number of queued refreshrequests queued in any one refresh control circuit for any bank. Themaximum number of queued refresh requests in combination with the largerpredetermined number of clock cycles prevent any worst case sequence ofexternal read or write cycles from causing more than two banks torefresh on the same clock cycle.

In one embodiment, the random access memory integrated circuit outputs arefresh flag to indicate that internal refresh requirements are notbeing met. The flag is output after a predetermined number of refreshrequests are queued. For example, the predetermined number of queuedrefresh requests may be at least two. In one embodiment, thepredetermined number may be almost as large as the predetermined numberof clock cycles separating the toggling of individual decoder outputs.

In one embodiment, each refresh control circuit includes a wrap aroundrefresh request counter configured to increment periodically, forexample, according to a number of clock cycles. The refresh controlcircuit further includes a wrap around refresh address counterconfigured to provide an address to be refreshed. The wrap aroundrefresh address counter increments once every time a refresh operationis carried out. Further, the refresh control circuit includes acomparator coupled to compare counts associated with addresses receivedfrom the wrap around refresh request counter and the wrap around refreshaddress counter. The comparator establishes a pending refresh upondetecting an inequality in the counts. The refresh control circuitfurther includes a circuit that, on any given cycle, determines a needor lack of need for each bank to do a memory access. If no memory accessis required, and the comparator indicates one or more pending refreshes,then that bank associated with the refresh control circuit will executeone refresh cycle. Further, a row address multiplexer coupled to thebank chooses a refresh address as the row address for which the bankwill execute a refresh. Thereafter, the wrap around refresh addresscounter increments. In one embodiment, the wrap around refresh requestcounter is permitted to count a plurality of counts ahead of the wraparound refresh address counter, thereby allowing queuing of a pluralityof refresh requests. The wrap around refresh request counter may be oneof a plurality of wrap around refresh request counters within the randomaccess memory integrated circuit, one for each bank of memory, theplurality of wrap around refresh request counters incrementing atdifferent times to stagger refresh operations of the plurality of bankswhereby the noise generated is reduced from that which would occur ifmore banks refreshed simultaneously. The count of each wrap aroundrefresh address counter provides an address to its respective bank, theaddress identifying a row of memory in that bank to be refreshed.

In one embodiment, each wrap around refresh request counter within eachrefresh control circuit receives an input from a decoder, the decoderoutput toggling once in response to a specific count of a wrap aroundclock cycle counter. The specific count of the wrap around clock cyclecounter allows the wrap around refresh request counter of that bank ofthe dynamic memory array to increment upon reaching the specific count.Each wrap around refresh request counter within each refresh controlcircuit receives a respective one of a plurality of the decoder outputsand increments its count when its respective decoder output toggles.Each refresh control circuit may include a wrap around refresh addresscounter incrementing in response to a refresh of the bank associatedthereto, an inequality between the count in the wrap around refreshrequest counter and the count in the wrap around refresh address counterassociated with a same bank indicating a pending refresh requirement forthat bank. The wrap around refresh address counter divides by a numberequal to the number of refresh cycles (such as 256) required to refreshall rows in a bank.

Another embodiment of the present invention is directed to a method ofrefreshing a dynamic memory array contained in a dynamic memory circuit,the dynamic memory array including one or more banks subject to aplurality of read and write accesses, the banks configured to hold datawithout requiring data duplication outside the banks other than datarequested and read from the dynamic memory array in one of the readaccesses which data is not yet output from the dynamic memory circuit.The method includes determining within the circuit containing thedynamic memory circuit when one of the banks within the dynamic memorycircuit requires a read or write access, and, if so, initiating anaccess to the dynamic memory without a delay associated with determiningwhether the data is available from a location outside the dynamicmemory, or whether there are any pending refresh requests, the read orwrite access having priority over any pending refresh requests. Further,the method includes determining within the circuit containing thedynamic memory circuit when one of the banks within the dynamic memoryarray requires a refresh and executing that refresh if no read or writeaccess is needed.

The method further includes retiring within one clock cycle one pendingrefresh request to one or more of the one or more banks having pendingrefresh requests if that bank(s) does not require an access on thatclock cycle.

One or more address bits choose the bank to be read or written. Themethod includes choosing a low order address bit, (e.g., a leastsignificant bit, a penultimate least significant bit, or bits thattoggle frequently) to select the bank to be read or written. The methodalso includes queuing refresh requests in a refresh control circuitcoupled to each bank. At least another embodiment includes a refreshcontrol circuit that generates refresh requests independent from anelapsed time determination, each refresh request generated as a functionof number of clock cycles received.

At least one embodiment of the method includes simultaneously performingaccess operations and refresh operations on different banks during thesame clock cycle. Each refresh control circuit includes a counterconfigured to increment to reflect a refresh request, the incrementingoccurring such that the counter coupled to only one of these banksincrements on any given clock cycle. Further, an embodiment limits thenumber of banks simultaneously performing access operations and refreshoperations to not more than three banks to prevent corruption of data bypower supply noise.

The present invention may be better understood, and its numerousobjects, features, and advantages made even more apparent to thoseskilled in the art by referencing the detailed description andaccompanying drawings of the embodiments described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a memory device incorporating a dynamicmemory array that provides an exemplary embodiment for describing thefeatures and advantages of the present invention.

FIG. 2 is a flow diagram describing a refresh method according to anembodiment of the present invention.

DETAILED DESCRIPTION Overview

An embodiment of the present invention is directed to a memory deviceincluding internal refresh control circuitry for automaticallyperforming refresh of the memory array without user intervention andalmost always without access interruption. A separate refresh controlcircuit is provided for each memory bank, each control being configuredto execute a refresh operation, for its respective memory bank, duringthe first available cycle following a refresh request which requestoccurs once every, for example, 256 external clock cycles.

Unlike most earlier devices, an embodiment is directed to “hidden”refresh cycles that can fully complete in just one clock cycle, allowinga read or write access cycle to begin in the same memory bank on thevery next clock cycle, if required, in accordance with a next commandand address received. Thus, even if the memory bank is free for only onecycle, a hidden refresh operation can be fully performed and the memorybank ready on the very next cycle to accept an external read or writecycle with full confidence. This makes hidden refresh cycles possibletotally under internal control, with no user interaction, because thereis no risk that an external cycle will be received that cannot beimmediately carried out because the memory bank is still busy performingan internal refresh cycle. For the same reason, there is no need for a“busy” signal to alert the user when an internal refresh cycle is inprogress.

Referring to FIG. 1, there are shown respective counters such as wraparound refresh request counters 122(0:3) within each refresh controlcircuit 150(0:3), the inputs to the respective wrap around refreshrequest counters being offset so that every 64 external (clock) cyclesone of the four memory banks (0:3) adds “1” to its number of queued orpending refresh requests. Whenever the number in the queue exceeds zero,that memory bank attempts a refresh on every cycle. Therefore, on thefirst available cycle after the queue is incremented, the request isretired (i.e., the refresh cycle is performed) and the count in thequeue is decremented. No further refresh requests are pending in thequeue when it decrements to its normal state of zero. The number ofqueued refresh requests for a bank is actually the number of counts thatthe wrap around refresh request counter 120(0:3) is ahead of a wraparound refresh address counter 128(0:3). For every 256 external clockcycles, as long as a memory bank is free for just one cycle, a hiddenrefresh can be performed without having the queued number of refreshrequests increase, except momentarily. In the memory device embodimentdescribed, two particular address bits (e.g., the A2 and A1 bit) choosewhich memory bank is addressed (in one particular implementation, theleast significant bit (LSB), address bit A0, chooses between the upperor lower 36-bit word of a 72-bit data field accessed from a singlememory bank).

In an exemplary embodiment, a user arranges the significance of theexternal address bits to use the same significance as the memory device;then, for any reasonable addressing sequence which accesses small orlarge blocks of memory, or even those which randomly address variousaddresses, the lower significance address bits are almost assuredlychanging or toggling frequently. Consequently, as the memory deviceresponds to the particular sequence of addresses, all four memory banks(0:3) are likely addressed with some reasonable frequency (thus ensuringthat all (other) memory banks are free reasonably often, such as, onaverage, 192 out of 256 cycles if all cycles are read or write cycles,and even more if some cycles are idle or burst cycles). The likelihoodof a single memory bank being busy for 256 straight cycles is very low.Moreover, the likelihood of a single memory bank being busy foradditional multiples of 256 straight cycles is extremely low.

A refresh request does not need to be retired before the next refreshrequest arrives. Up to 63 refresh requests may be placed into therefresh queue. A refresh flag is provided in the extremely unlikelyevent that the internal refresh for a particular memory bank gets veryfar behind its desired rate (i.e., if too many refresh requests arequeued). If the refresh queue is nearly full, the refresh flag isasserted. For example, if a particular memory bank that previously hadno queued refresh requests has not performed a refresh cycle in 62×256external clock cycles (because all those cycles required a read or writeaccess to that bank), the refresh flag is asserted. However, even if amemory bank gets seriously “behind” and has queued up many refreshrequests, if the memory bank thereafter is free for more than one cycleout of every 256 external clock cycles, refresh requests will be retiredfrom the queue faster than new ones will be added, and the device willeventually “catch up.” It is for this reason, along with the depth ofthe refresh queue, that the refresh flag is seldom, if ever, expected tobe asserted (provided the user, for example, assigns the significance ofthe address bits as described).

In one embodiment, the refresh counters and control circuits arearranged to ensure that no more than three memory banks can perform aninternal array operation at the same time (one performing a read orwrite cycle, and two each performing a hidden refresh cycle) to reducethe worst case current transients (i.e., power supply noise) that wouldoccur if all four memory banks shown in FIG. 1 (or more than four memorybanks in other embodiments) were simultaneously active.

The choice of counting clock cycles to determine when to refresh, ratherthan elapsed time, has several advantages. First, it is easy to do. Theclock exists and is always running (as required by the phase-lockeddelay line described in patent application Ser. No. 09/503,048,referenced above and incorporated herein for all purposes). Second, therefresh interval required for proper operation is more or lessproportional to the operating cycle time. As the operating frequencyincreases (i.e., cycle time decreases), more minority carriers areinjected into the substrate, increasing the leakage of the memory cells,thus decreasing the data retention time of the memory cells.Consequently, faster cycling (i.e., shorter cycle times) requires morefrequent refreshing. Furthermore, another significant source of cellleakage is the sub-threshold conduction of the access transistors onunselected rows of memory cells. This sub-threshold conduction issignificant only when the bit line (or complement bit line), to whichthe unselected memory cell is attached, is biased at VSS, which onlyoccurs for a fixed time of about 2 ns per cycle. This component ofmemory cell leakage is directly proportional to frequency. Again, higherfrequency operation requires more frequent refreshing.

Structure

Turning now to FIG. 1 more particular embodiments of the above-describedrefresh system are described. FIG. 1 shows a block diagram of a randomaccess memory integrated circuit illustrating circuitry appropriate forthe refresh operations described. Random access memory integratedcircuit 100 includes input address block 102, read/not write input block103, access request input block 104, clock input block 105, writeaddress queue 110, read/write address multiplexer 112, input address bus106, master control block 114, divide-by 256 wrap around clock cyclecounter 116, clock cycle count decoder 118, refresh control circuits150(0:3), and dynamic memory banks 140(0:3). In one or more embodiments,the input address block 102, read/not write input block 103, accessrequest input block 104, and clock input block 105 receive signals frombonding pads.

Master control block 114 receives a clock signal from clock input block105 and a variety of other control signals including a read/not writecontrol from read/not write input block 103, an access request inputfrom access request input block 104, and an input address via addressbus 107. The master control block 114 prioritizes the read or writeaccess requests over pending memory refresh requests. Read accessrequests are initiated without first determining whether data isavailable from outside the dynamic memory array, e.g., from an internalcache memory, thus avoiding a delay associated with such determination.Address bus 106 couples the input address block 102 to write addressqueue 110 (whose function is described in U.S. patent application Ser.No. 09/503,048, incorporated herein for all purposes) and read/writeaddress multiplexer 112. Address bus 107 couples the read or writeaddress output from address multiplexer 112 to refresh control circuits150(0:3). Control bus 109 couples the master control 114 to the bankread/write/refresh controllers 120(0:3).

Random access memory integrated circuit 100 further illustrates aplurality of dynamic memory banks 140(0:3). Each of the dynamic memorybanks 140(0:3) includes a bank row decoder 130(0:3), and a correspondingbank dynamic memory array 132(0:3), respectively. Bank row addressbusses 145(0:3) couple each of the dynamic memory banks 140(0:3) to adedicated row address from a dedicated address multiplexer 126(0:3).Refresh control circuits 150(0:3) periodically refresh dynamic memoryarrays 140(0:3) respectively, to maintain data.

The dynamic memory array, including each bank dynamic memory array132(0:3) within each dynamic memory bank 140(0:3), receive read andwrite access requests. According to an embodiment, the read or writeaccess requests have priority over pending refresh requests. Further,one pending refresh request for a bank retires on a clock cycle notrequiring an access of that dynamic memory bank. Refresh cycles, likeaccess cycles, complete in a single clock cycle. The read accessrequests initiate an access to the dynamic memory array without firstdetermining whether data is available from outside the dynamic memoryarray, thus avoiding a delay associated with such a determination.Because each memory bank 140(0:3) has a dedicated refresh controlcircuit 150(0:3), one dynamic memory bank may execute a memory bankaccess while other banks may simultaneously execute refresh operations.

Referring to FIG. 2, flow diagram 200 illustrates a method forrefreshing a dynamic memory array when one (or more) of the banks in thedynamic memory array requires a refresh. During operation of a dynamicrandom access memory integrated circuit, such as that shown in FIG. 1,block 210 determines when a bank, such as dynamic memory banks 140(0:3)within a dynamic memory array requires a refresh. Block 220 prioritizesread and write access requests over any pending refresh request. Inparticular, the read and write access requests initiating an access tothe dynamic memory array occur without a delay associated withdetermining whether data is available from a location outside thedynamic memory array or whether refresh requests are pending. Block 230shows that the random access memory integrated circuit selects the bank(if any) to be read or written according to predetermined bits of theaccess address. Block 240 shows that during any clock cycle in which amemory bank is not selected for a read or write operation, and in whichthat bank has one or more pending refresh requests, one refreshoperation is executed.

Returning to FIG. 1, each dynamic memory bank of memory banks 140(0:3)has an independent row address bus 145(0:3). A memory bank requiringread or write access has the access address while other memory banksrequiring refresh have their respective refresh addresses. The bank rowaddress multiplexers 126(0:3) use at least one bit of the read or writeaccess address (and the access request) to select the bank for read orwrite access. The row address multiplexers 126(0:3) receive the bit(s)of the address selecting dynamic memory banks 140(0:3) for read or writeaccess via address bus 107. Preferably, one or more of the lower orderaddress bits select the memory bank for read or write access in randomaccess memory integrated circuit 100. The address bit(s) that are usedto select the memory bank 140(0:3) for read or write access, in oneembodiment, are designed to change frequently, so that excessiveaccessing does not prevent a memory bank from meeting refreshrequirements. Choosing a low-order address bit to select banks helpscause the memory banks 140(0:3) to share read and write accessing toprevent one bank from receiving excessive accesses. Further, in randomaccess memory integrated circuit 100, excessive accessing of a singlebank may include read or write accesses exceeding, on average, over 255clock cycles of every 256 clock cycles averaged over many thousands ofclock cycles. That is, on average, each bank is expected to be free toretire a refresh request on at least 75% (at most only one of the fourbanks is accessed in any cycle and at least three of four banks are freeto be refreshed) of clock cycles but can keep up with its refreshrequirements even if it is free only 0.4% (1 of 256) of clock cycles. Asone of ordinary skill in the art appreciates, the percentages willchange according to design requirements, such as the number of banks inthe circuit. A refresh is required once every predetermined number ofclock cycles. In one embodiment, the predetermined number of clockcycles is 256 cycles. The number of clock cycles is a function of therefresh requirements for the dynamic memory array. Wrap around clockcycle counter 116 generates a count that increments once for every clockcycle returning to count 0 after a predetermined number of clock cycles,for example, each 256 clock cycles. The number of clock cycles may beaccording to the number of rows in the dynamic memory array, andaccording to the refresh requirements of the dynamic memory array.

Decoder 118 receives the count generated by wrap around clock cyclecounter 116. Decoder 118 decodes counts 0, 64, 128 and 192. Each ofthese four decoder outputs toggles once every 256 clock cycles, but toreduce peak power consumption and noise, they do not toggle on the samecycle. More particularly, in a preferred embodiment, the number of clockcycles separating the decoder counts from decoder 118 is more than themaximum number of queued refresh requests queued for any bank. By havingthe maximum number of queued refresh requests for any bank be less thanthe number of clock cycles between cycles incrementing the plurality ofwrap around refresh request counters 122(0:3) of the various banks140(0:3), no matter how many banks are coupled to receive the toggledoutputs from decoder 118, no more than two banks will ever perform arefresh on a given cycle. That is, no sequence of external read or writecycles (that do not cause the refresh flag to remain asserted for morethan a few cycles) can cause more than two banks to refresh on the sameclock cycle.

In an embodiment, each of the refresh control circuits 150(0:3) iscoupled to receive its respective output from decoder 118. The refreshcontrol circuits 150(0:3) inputs (decoder outputs) toggle apredetermined number of clock cycle counter counts apart, thepredetermined number being greater than the maximum number of refreshrequests permitted to be queued in any one refresh control circuit150(0:3). In one embodiment, up to 62 refresh requests may be queued inany one refresh control circuit 150(0:3). Refresh control circuits150(0:3) generate refresh requests. Each refresh control circuit150(0:3) includes a read/write/refresh controller 120, a wrap aroundrefresh request counter 122, a row address comparator 124, a row addressmultiplexer 126, and a wrap around refresh address counter 128. Wraparound refresh request counters 122(0:3) generate one refresh request(increment one count) upon receiving an input from decoder 118, theinput being received at a particular count of wrap around clock cyclecounter 116. The wrap around refresh request counters 122(0:3) in thebank refresh control circuits 150(0:3) increment at different times (indifferent cycles) to stagger the refresh operations of the banks140(0:3). The staggering of refresh operations generates less noise thanif all of the memory banks refreshed simultaneously. The wrap aroundrefresh request counters 122(0:3) increment in response to therespective refresh signals generated by decoder 118. If it should everoccur (extremely unlikely) that a predetermined number of unsatisfiedrefresh requests are queued in any bank, that bank refresh controlcircuit 150(n) will generate a flag signal (not shown in FIG. 1) toindicate that internal refresh requirements are not being met. This flagis an output of random access memory integrated circuit 100. In oneembodiment, the predetermined number of queued refresh requests isgreater than two, allowing more than one refresh request to be queuedbefore the flag is generated.

Decoder 118 periodically toggles one of its outputs (0, 64, 128, 192) toperiodically generate refresh requests for its respective dynamic memorybank 140. Wrap around refresh request counters 122(0:3) each incrementonce for each refresh request to its respective dynamic memory bank140(0:3). For example, referring to dynamic memory bank(1) assume that aclock cycle causes wrap around cycle counter 116 to advance to count 64.Wrap around refresh request counter 122(1) increments to a new count fordynamic memory bank 140(1).

The new count in bank 1 wrap around refresh request counter 122(1) fornow differs from the address (count) in bank 1 wrap around refreshaddress counter 128(1). Bank 1 row address comparator 124(1) detectslack of equality and outputs a “refresh pending” signal to bank 1read/write/refresh controller 120(1). This “refresh pending” cycle willlast until the first cycle for which bank 1 is not busy doing, forexample, a (higher priority) read or write access. More particularly,the refresh is immediately performed if bank 1 is not otherwise occupiedwith a read or write cycle (i.e., a normal read or write cycle or aburst read or write cycle to an internally generated address to continuea burst read or write). If the memory bank 140(1) happens to be busywith such a read or write cycle and is therefore unable to perform therefresh when requested, the request is queued and performed the firstcycle that the memory bank is available (i.e., not executing a read orwrite cycle).

During the first such available cycle, the pending refresh will beretired. Specifically, bank 1 wrap around refresh address counter 128(1)provides the row address to be refreshed to bank 1 row addressmultiplexer 126(1), bank 1 row address multiplexer 126(1) then selectsthat address to place on bank 1 address bus 145(1), and bank 1read/write/refresh controller 120(1) executes the refresh operation.After executing the refresh operation, bank 1 read/write/refreshcontroller 120(1) increments bank 1 wrap around refresh address counter128(1) to the next count (row address) to be refreshed in that bank(here, bank 1).

With bank 1 wrap around refresh address counter 128(1) incrementing, thecount of bank 1 wrap around refresh address counter 128(1) and the countof bank 1 wrap around refresh request counter 122 (1) now match. Thus,bank 1 row address comparator 124(1) outputs no present pending refreshrequests.

In the very unlikely event that there are no available cycles to do therefresh (as when all cycles require bank 1 accesses), wrap aroundrefresh request counter 122(1) may get many counts ahead of bank 1 wraparound refresh address counter 128(1). Then, the refresh requests may beretired in sequential cycles or otherwise as the lack of read/writeactivity to Bank 1 permits. If Bank 1 (or any bank) is so busy servicingaccess requests that fewer than one cycle per 256 (for example) clockcycles is available to retire refresh requests, and this conditioncontinues until the count of wrap around refresh request counter 122(1)is ahead of the count of wrap around refresh address counter 128(1) by alarge predetermined number of counts, then, a “refresh needed” flag (notshown) is raised by the row address comparator for that bank and by therandom access integrated circuit 100 indicating that the circuit is notable to service its internal refresh requirements and that some cyclesmust soon be made available by the system that do not require read orwrite access (to Bank 1 in this case).

In general, a wrap around refresh address counter 128(n) (the nrepresents any one of the functional blocks shown in the four memorybank circuits 0:3, as each memory bank may perform refresh functions atdifferent times) increments in response to a refresh of dynamic memorybank 140(n). Row address comparator 124(n) compares the count in thewrap around refresh request counter 122(n) with the count in the wraparound refresh address counter 128(n). When these counts are unequal,row address comparator 124(n) generates a signal indicating a pendingrefresh requirement for dynamic memory bank 140(n). The signal generatedby row address comparator 124(n) is received by read/write/refreshcontroller 120(n). In addition, row address comparator 124(n) may asserta refresh flag when the count of wrap around refresh request counter122(n) exceeds the count of wrap around refresh address counter 128(n)by a predetermined number. Wrap around refresh address counter 128(n)provides row address multiplexer 126(n) with a row address for refresh.The output of row address multiplexer 126(n) is coupled to anindependent row address bus 145(n) dedicated to dynamic memory bank140(n). Independent row address busses 145(n) allow at least one bank toperform a refresh operation of a refresh address and another bank toperform an access operation of an access address in the same clockcycle.

The many aspects, features, and advantages of the present invention areconveyed herein by describing several exemplary embodiments of ahigh-performance dynamic random access memory (DRAM) memory device.Simplified block diagrams are shown to more easily communicate keyconcepts, features, or implementation details. One skilled in the artwill recognize the many teachings of this disclosure and be able toapply these teachings to additional embodiments and, indeed, to otherkinds of devices, as well, without departing from the teachings of thisdisclosure. Consequently, the scope of the invention should not beinferred as being limited by the exemplary embodiments described herein,but rather should be viewed as teaching in the art far greater than justthese exemplary embodiments. Accordingly, other embodiments, variations,and improvements not described herein are not necessarily excluded fromthe scope of the invention.

1. A random access memory integrated circuit responsive to an externallysupplied clock input, the random access memory integrated circuitcomprising: a dynamic memory array configured in one or more banks,wherein the dynamic memory array requires periodic refreshing tomaintain data; and one or more refresh control circuits generatingrefresh requests inside the random access memory integrated circuit, thedynamic memory array configured to receive read and write accessrequests, wherein the read or write access requests have priority overpending refresh requests, wherein one pending refresh request to one ofthe banks is retired on any clock cycle not requiring an access of thatbank, the refresh completing in the clock cycle, the read accessrequests initiating an access to the dynamic memory array without firstdetermining whether data is available from outside the dynamic memoryarray, thereby avoiding a delay associated with such determination. 2.The random access memory integrated circuit of claim 1 wherein each ofthe one or more banks has a one to one correspondence with one of theone or more refresh control circuits, each refresh control circuitcontrolling refresh of the bank coupled thereto.
 3. The random accessmemory integrated circuit of claim 1 wherein: the dynamic memory arrayis configured in two or more banks, each of the two or more banks havinga one to one correspondence with one of two or more refresh controlcircuits, each refresh control circuit controlling refresh of the bankcoupled thereto; and wherein during the clock cycle for which one bankof the dynamic memory array executes a read or a write operation, atleast another bank may execute a refresh operation during the same clockcycle.
 4. The random access memory integrated circuit of claim 3 whereinduring the clock cycle for which one bank executes a read or a writeoperation, two other banks may simultaneously execute refreshoperations.
 5. The random access memory integrated circuit of claim 3further comprising: an address bus coupled to each of the banks, each ofthe banks configured to receive an independent address, wherein at leastone bit of a read or write access address is used to select the bank forread or write access.
 6. The random access memory integrated circuit ofclaim 5 wherein the bit is selected from among low-order address bits.7. The random access memory integrated circuit of claim 5 wherein thebit changes state frequently.
 8. The random access memory integratedcircuit of claim 5 wherein the choice of the bit of the address allowsthe banks to share read and write accessing such that no one bankrequires excessive accessing, the sharing of the read and writeaccessing thereby allowing each bank to meet refresh requirements. 9.The random access memory integrated circuit of claim 8 wherein excessiveaccessing includes read or write accessing more than 255 clock cycles ofeach 256 clock cycles.
 10. The random access memory integrated circuitof claim 3, wherein each refresh control circuit is configured toreceive an input from a decoder, the decoder producing one refreshrequest after a predetermined number of external clock cycles, therefresh requests generated independent of an elapsed time counter. 11.The random access memory integrated circuit of claim 3, wherein each ofthe refresh control circuit inputs is coupled to receive an output froma decoder, the decoder coupled to receive an input from a wrap aroundclock cycle counter.
 12. The random access memory integrated circuit ofclaim 11 wherein the decoder is a counts 0, 64, 128, 192 decoder and thewrap around clock cycle counter is a divide by 256 wrap around clockcycle counter.
 13. The random access memory integrated circuit of claim11 wherein the wrap around clock cycle counter increments once everyclock cycle and provides an input to the decoder, the wrap around clockcycle counter in combination with the decoder causing the decoder tosequentially provide a plurality of outputs, each output configured tobe received by one of the refresh control circuits, the decoderproviding decoded counts that are a specific number of clock cyclesapart.
 14. The random access memory integrated circuit of claim 13wherein each output is received by a designated wrap around refreshrequest counter.
 15. The random access memory integrated circuit ofclaim 13 wherein the specific number of clock cycles is more than amaximum number of queued refresh requests queued in any one refreshcontrol circuit for any bank.
 16. The random access memory integratedcircuit of claim 15 wherein the maximum number of queued refreshrequests in combination with the specific number of clock cyclesprevents a sequence of external read or write cycles from causing morethan two banks to refresh on the same clock cycle.
 17. The random accessmemory integrated circuit of claim 3 wherein each refresh controlcircuit includes: a wrap around refresh request counter that incrementsfor each refresh request; and a wrap around refresh address counter thatincrements in response to a refresh of a bank associated thereto; and arow address comparator coupled to the wrap around refresh requestcounter and to the wrap around refresh address counter, an inequalitybetween a count in the wrap around refresh request counter and a countin the wrap around refresh address counter indicating a pending refreshrequirement for the bank associated with the refresh control circuit.18. The random access memory integrated circuit of claim 17 wherein thewrap around refresh request counter is one of a plurality of wrap aroundrefresh request counters within the random access memory integratedcircuit, the plurality of wrap around refresh request countersincrementing at different times to stagger refresh operations of thebanks whereby the noise generated is reduced from that which would occurif more banks refreshed simultaneously.
 19. The random access memoryintegrated circuit of claim 18 wherein each of the wrap around refreshrequest counters is coupled to a decoder, each wrap around refreshrequest counter receiving a different output from the decoder, thedifferent decoder outputs being responsive to different counts indifferent cycles thereby causing the incrementing of the plurality ofwrap around refresh request counters at different times to stagger therefresh operations of the banks.
 20. The random access memory integratedcircuit of claim 3 further comprising: a row address multiplexer coupledto an address bus, the row address multiplexer receiving at least onebit of a read or write access address to select a bank of the one ormore banks for a read or write access.
 21. The random access memoryintegrated circuit of claim 3 wherein each refresh control circuit iscoupled to a wrap around clock cycle counter having an output that isdecoded, wherein each decoded output toggles once for everypredetermined number of clock cycles and is provided to a wrap aroundrefresh request counter within each refresh control circuit, the wraparound refresh request counter coupled to receive the decoded output ofthe wrap around clock cycle counter and to increment one count each timethe decoder output toggles, each refresh control circuit furtherincluding a wrap around refresh address counter incrementing in responseto a refresh of the bank associated thereto, an inequality between acount in the wrap around refresh request counter and a count in the wraparound refresh address counter associated with a same bank indicating apending refresh requirement for that bank.
 22. The random access memoryintegrated circuit of claim 21 wherein the wrap around refresh addresscounter divides by a number equal to the number of cycles required torefresh all rows in the bank associated thereto.
 23. The random accessmemory integrated circuit of claim 22 wherein the number equal to thenumber of cycles is
 256. 24. The random access memory integrated circuitof claim 21 wherein the wrap around refresh request counters areassociated with different banks, the wrap around refresh requestcounters incrementing at different times to stagger refresh operationsof the banks whereby the noise generated is reduced from that whichwould occur if more banks refreshed simultaneously.
 25. The randomaccess memory integrated circuit of claim 21 wherein the count of thewrap around refresh address counter provides an address to itsrespective bank, the address identifying a row of memory in that bank tobe refreshed.
 26. The random access memory integrated circuit of claim21 wherein when the count of the wrap around refresh request counter ofany bank exceeds the count of the wrap around refresh address counter ofthe corresponding bank by a predetermined number, a refresh flagasserts.
 27. The random access memory integrated circuit of claim 21wherein the inequality for each bank is determined by a row addresscomparator.
 28. The random access memory integrated circuit of claim 3,wherein each bank is coupled to one of the refresh control circuits, andeach bank is coupled to an independent row address bus thereby allowingat least one bank to perform a refresh operation and another bank toperform an access operation in the same clock cycle.
 29. The randomaccess memory integrated circuit of claim 1 wherein the random accessmemory integrated circuit outputs a refresh flag to indicate thatinternal refresh requirements are not being met.
 30. The random accessmemory integrated circuit of claims 29 wherein the flag is output aftera predetermined number of refresh requests are queued.
 31. The randomaccess memory integrated circuit of claim 30 wherein the predeterminednumber of queued refresh requests is at least two.
 32. The random accessmemory integrated circuit of claim 30 wherein the predetermined numberof queued refresh requests is less than but close to a specific numberof clock cycles separating a toggling of a plurality of outputs from adecoder, each output from the decoder configured to be received by oneof the refresh control circuits.
 33. The random access memory integratedcircuit of claim 1 wherein each refresh control circuit includes: a wraparound refresh request counter that increments for each refresh request;and a wrap around refresh address counter that increments in response toa refresh of a bank associated thereto; and a row address comparatorcoupled to the wrap around refresh request counter and to the wraparound refresh address counter, an inequality between a count in thewrap around refresh request counter and a count in the wrap aroundrefresh address counter indicating a pending refresh requirement for thebank associated with the refresh control circuit.
 34. The random accessmemory integrated circuit of claim 33 wherein the count of the wraparound refresh address counter provides an address to its respectivebank, the address identifying a row of memory in that bank to berefreshed.
 35. The random access memory integrated circuit of claim 34wherein each refresh control circuit includes a row address multiplexer,the row address multiplexer receiving the address identifying a row ofmemory in the bank to be refreshed and receiving a read or write addressfor a read or write access, the output of the row address multiplexerbeing a read or write address if a read or write access is required ofthat bank and, when a read or write access is not required and a refreshis required, the output of the row address multiplexer being a refreshaddress received from the wrap around refresh address counter.
 36. Therandom access memory integrated circuit of claim 34 wherein the outputof the wrap around refresh address counter is coupled to a row addressmultiplexer, the row address multiplexer configured to send the addressidentifying a row of memory in that bank to be refreshed if the bank isnot performing a read or write access.
 37. The random access memoryintegrated circuit of claim 33 wherein when the count of the wrap aroundrefresh request counter of any bank exceeds the count of the wrap aroundrefresh address counter of the corresponding bank by a predeterminednumber, a refresh flag asserts.
 38. The random access memory integratedcircuit of claim 37 wherein the predetermined number is
 62. 39. A randomaccess memory integrated circuit comprising: a dynamic memory arrayconfigured in one or more banks, wherein the dynamic memory arrayrequires periodic refreshing to maintain data; and one or more refreshcontrol circuits generating refresh requests inside the random accessmemory integrated circuit, the dynamic memory array configured toreceive read and write access requests, wherein the read or write accessrequests have priority over pending refresh requests, wherein onepending refresh request to one of the banks is retired on a clock cyclenot requiring an access of that bank, each refresh completing in theclock cycle, each access request reading data from or writing data tothe dynamic memory array, whereby data is never read from or written toan on-board cache memory, thereby saving the area that the on-boardcache memory would occupy.
 40. The random access memory integratedcircuit of claim 39 wherein the dynamic memory array is configured intwo or more banks, each of the two or more banks having a one to onecorrespondence with one of two or more refresh control circuits, eachrefresh control circuit controlling refresh of the bank coupled thereto.41. The random access memory integrated circuit of claim 40 whereinduring the clock cycle for which one bank of the dynamic memory arrayexecutes a read or a write operation, at least another bank may executea refresh operation during the same clock cycle.
 42. The random accessmemory integrated circuit of claim 41 wherein during the clock cycle forwhich one bank executes a read or a write operation, two other banks maysimultaneously execute refresh operations.
 43. The random access memoryintegrated circuit of claim 41 further comprising: an address buscoupled to each of the banks, wherein each of the address busses areconfigured to receive an independent address.
 44. The random accessmemory integrated circuit of claim 43 wherein at least one bit of a reador write access address is used to select the bank for read or writeaccess.
 45. The random access memory integrated circuit of claim 44wherein the bit is selected from among low-order address bits.
 46. Therandom access memory integrated circuit of claim 44 wherein the bitchanges state frequently.
 47. A method of refreshing a dynamic memoryarray contained in a circuit, the dynamic memory array including one ormore banks subject to a plurality of read and write accesses, the banksconfigured to hold data without requiring data duplication outside thebanks other than data transferred from the dynamic memory array in oneof the read accesses, the method comprising: determining within thecircuit containing the dynamic memory array when one of the banks withinthe dynamic memory array requires a refresh; prioritizing read or writeaccess requests over any pending refresh requests, wherein read accessrequests initiating an access to the dynamic memory array occur withouta delay associated with determining whether the data is available from alocation outside the dynamic memory array; and retiring, within oneclock cycle, one pending refresh request to a bank when that bank haspending refresh requests and does not also require an access of thedynamic memory array on that clock cycle.
 48. The method of claim 47wherein the dynamic memory array includes two or more banks, the methodfurther comprising: controlling the refreshing with two or more refreshcontrol circuits contained in the circuit, each bank controlled with therefresh control circuit coupled thereto, each refresh control circuitchecking whether its respective bank is subject to a read or writeaccess, the refresh control circuit retiring a pending refresh when itsrespective bank is not subject to the read or write access.
 49. Themethod of claim 48 wherein there is a one to one correspondence betweeneach bank and each refresh control circuit, each refresh control circuitcontrolling the refreshing of the bank coupled thereto.
 50. The methodof claim 48 further comprising: selecting the bank to be read or writtenaccording to predetermined bits of a read or write access addressreceived via an address bus coupled to each of the refresh controlcircuits.
 51. The method of claim 50 wherein the predetermined bitsinclude one of low order address bits, a least significant bit, apenultimate least significant bit, and bits that toggle frequently. 52.The method of claim 48 further comprising: performing access operationsand refresh operations on different banks during the same clock cycle.53. The method of claim 52 wherein the number of banks performing accessoperations and refresh operations is limited to less than all banks toprevent corruption of data by power supply noise.
 54. The method ofclaim 48 wherein each refresh control circuit includes a wrap aroundrefresh request counter configured to increment to reflect a refreshrequest, the incrementing occurring such that only the wrap aroundrefresh request counters coupled to one of the banks increment on anygiven clock cycle.
 55. The method of claim 48 wherein each of the two ormore banks is configured to receive a respective input from itsrespective refresh control circuit.
 56. The method of claim 48 whereineach refresh control circuit receives an output from a decoder, theoutput of the decoder being received by a wrap around refresh requestcounter within each refresh control circuit and the decoder beingconfigured to receive an input from a wrap around clock cycle counter.57. The method of claim 56 wherein the wrap around clock cycle counterprovides an input to the decoder, wherein every predetermined number ofclock cycles, the wrap around clock cycle counter in combination withthe decoder cause the decoder to sequentially toggle one of a pluralityof outputs from the decoder, each of the plurality of outputs beingreceived by a respective one of the refresh control circuits.
 58. Themethod claim 57 wherein: each output of the plurality of outputs isreceived by one of the wrap around refresh request counters; and thepredetermined number of clock cycles is more than a maximum number ofqueued refresh requests that can be queued in each refresh controlcircuit.
 59. The method of claim 58 wherein the maximum number of queuedrefresh requests in combination with the predetermined number of clockcycles prevent a sequence of external read or write accesses fromcausing more than two banks to refresh on a same clock cycle.
 60. Themethod claim 57 wherein the predetermined number of clock cycles is morethan a maximum number of queued refresh requests within the wrap aroundrefresh request counter in any one refresh control circuit for any ofthe banks.
 61. The method of claim 60 wherein the predetermined numberof clock cycles prevents more than two banks of the two or more banksfrom performing a refresh at the same time.
 62. The method of claim 47further comprising: queuing refresh requests in a refresh controlcircuit coupled to each bank.
 63. The method of claim 47 wherein therefresh control circuit generates refresh requests independent from anelapsed time determination, each refresh request generated as a functionof a number of clock cycles received.